INTERNAL CRACK INITIATION IN HIGH CYCLE FATIGUE AT CRYOGENIC TEMPERATURES

High cycle fatigue properties at cryogenic temperatures have been investigated for various structural materials like titanium alloys, high manganese steels, and austenitic stainless steels. Some of the test materials show internal crack initiation which is not associated with pores or inclusions. The internal crack initiation occurs in low peak stress tests, although the surface initiation occurs in high peak stress tests. This internal initiation, however, has microstructural origins as follows: a duplex structure phase with low Al content for a Ti-5Al-2.5Sn alloy, alpha grain for Ti-6Al-4V alloys, Mo-rich zone for a cold-rolled SUS316LN steel, and grain boundary crack for 0.1N-32Mn-7Cr steels. In Ti-6Al-4V alloys, the size of initiation site depends on the peak stress. Transmission electron microscopy demonstrates heterogeneous dislocation structure in fatigued specimens with internal cracks for the Ti-5Al-2.5Sn alloy and the 32Mn-7Cr steel. One possible mechanism of internal initiation is microcracking in weak structures by stress or strain concentration due to localized deformation.